Contact image sensor (CIS) technology is employed in image reading devices such as copiers and scanners. An image reading device with the CIS technology includes gradient index rod lenses and imaging elements arranged in a line. Light is scattered on a scan target object, passes through the gradient index lenses, and forms an image on the imaging elements. The image reading device is used for scanning various types of documents. When the document is a paper sheet, the whole document can be on a glass surface during scanning. When the document is a bound document, however, the bound area of the document is away from the glass surface during scanning. When gradient index lenses are used, the image reading device may fail to scan the scan target object at a sufficiently high resolution if the scan target object is partly away from the glass surface of the image reading device.
Because the refractive index varies with the wavelength of light, the light having passed through the lens has different focus positions corresponding to color components represented by red (R), green (G), and blue (B). Thus, when the document is positioned at the focus position of the G color component, for example, the position of the document is out of focus positions of the R and B color components, resulting in blurred images corresponding to the R and B color components. In other words, an image reading device employing a lens causes axial chromatic aberration, which is a phenomenon in which different wavelengths of light form images at different positions.
The image reading device disclosed in Patent Literature 1 scans beforehand patterns that represent concentrated ideal point light sources on a provided reference image, and calculates a point spread function (PSF) for each of R, G, and B. The image reading device corrects chromatic aberration by creating correction filters based on the calculated PSFs and applying the correction filters to the scanned image to correct for each of R, G, and B.